Your first issue is very strange, I have not seen that and can't replicate it here. If it is possible for you to share a screenshot or better yet a short video of the issue occurring that would be great. You can send it to support AT
planetside.co.uk, or put it up privately on YouTube, perhaps.
The second issue makes me wonder if there is some negative interaction happening with another piece of software or hardware on your system. That kind of responsiveness issue is definitely not normal. You describe this issue as if it always happens to you, regardless of the scene. Is that the case? Does it occur even with the default scene you get when you start Terragen?
You mention your stylus and it seems like one of the few elements in your setup which may be less common. Would it be possible for you to try disabling/disconnecting your stylus to see if that has any effect?
The other thing that comes to mind is some kind of graphics system incompatibility. What version of OS X are you running and what specific model is your iMac? Can you try a test for us? When you have Terragen open and it is having this "Mac rainbow" wait icon behavior, try *closeing* the 3D preview with the X button upper-right of that part of the UI. Then try switching between layouts (e.g. Atmosphere to Terrain, as you mentioned) and see if there is the same delay.
- Oshyan